This invention relates to the formation of films from sheets of polymeric materials. More particularly, it relates to the production of polymer films by the continuous cold rolling of thermoplastic polymer in sheet form.
Films made from polymer materials, and particularly from synthetic organic thermoplastic polymers such as polyethylene and polypropylene, have found widespread utility in such diverse fields as packaging, construction, magnetic tape recording and photography. However, there has been a long-felt need among film processors and users for polymer films having improved physical properties such as strength, stiffness and clarity.
Various methods have been developed in the past for enhancing the physical properties of already-formed (e.g., cast, extruded or skived) films. For example, the film may be fed into a quenching bath immediately after having been formed by melt-extrusion or casting. In addition, films may be stretched in one or more directions or calendered at temperatures above the softening point or range of the polymeric material by means of heated rollers. However, these post-formation procedures for improving the characteristics of polymer films have drawbacks which limit their usefulness in many cases. Thus, stretching methods tend to enlarge any pin holes or voids which may be present in the polymer film as originally prepared, thereby decreasing the moisture-barrier properties of the film and diminishing its usefulness for many packaging purposes. Moreover, calendering often fails to achieve the degree of property enhancement desired, particularly with regard to film clarity and color uniformity.
Another method which has been investigated with a view toward the processing of pre-formed polymer films, and to which the present invention is directed, involves the compression rolling of thermoplastic sheet material which in effect extends and orients the polymer molecules within the latter. Previous efforts toward developing processes for the compression rolling of plastic are described in Williams et al. SPE Journal 17, 42-48 (1971) and in U.S. Pat. Nos. 3,504,075, 3,194,863, 3,083,410, and Re. 27,404. These methods generally involve "full fluid rolling" (i.e., rolling with the use of a layer of lubricant between the film and roller surfaces). In this technique, the surfaces of the plastic sheet material and the rollers at the "nip" (i.e., the point at which compression actually takes place) are separated by the lubricant which forms a "hydrodynamic wedge" between the rollers and the sheet material in front of the nip as the material passes between the rollers.
In hydrodynamic or full fluid lubrication the surfaces in relative motion (i.e., the work roll surfaces and polymeric sheet material) are separated at all times by a continuous uninterrupted fluid lubricant layer so that at no time is there actual physical contact between opposing surfaces. In practice, however, it is often impossible or disadvantageous to maintain a continuous plastic film rolling operation under hydrodynamic lubrication conditions. Thus, hydrodynamic lubrication is limited by the adverse effect of the applied loads or pressures. An increase in the applied load, a frequent requirement for achieving a desired degree of reduction in the polymeric film with resultant improvements in the film clarity and physical properties, requires a compensating increase in the viscosity of the fluid lubricant and/or an increase in the rolling speed. Inasmuch as increments of applied load may require proportionately much larger increases in the fluid viscosity and/or rolling speed, the compression rolling of polymeric films under conditions of hydrodynamic lubrication imposes serious practical limitations in successfully compression rolling under high roll pressures. Thus, increasing the rolling speed causes the generation of unwanted heat as a result of the additional work done on the fluid film. On the other hand, for a given rolling speed and applied load, there is only one optimum value of the lubricant viscosity under hydrodynamic conditions.
Accordingly, it is an object of the present invention to provide an improved process for extending and orienting a preformed polymer sheet by compression rolling.
Another object is to provide an improved process for the compression rolling of cast, extruded, or skived thermoplastic sheet material to effect a reduction in thickness thereof to form a film having improved properties including enhanced strength, stiffness and clarity.
These and other objects of the invention, as well as a fuller understanding of the advantages thereof, can be had by reference to the following detailed description and claims.